Carbamazepine is an effective and generally safe medication for complex partial and generalized tonic clonic seizures in adults. Nevertheless, the relationships between dose and plasma concentration varies widely among patients and therefore the relationship between drug levels and toxicity, as well as seizure control, is non-existent in patients who are taking other anti-epileptics. We are evaluating the relative importance of carbamazepine and its 10,11-epoxy metabolite in assessing those objectives. We have developed a simple and rapid quantitation procedure for measuring the parent compound and its metabolite, in presence of other anti-convulsants (e.g. phenytoin and valproic acid) using a high-pressure liquid chromtography system and UV detection. Four patients have been extensively studied. We are in the process of correlating drug and/or metabolite levels with seizure frequency and toxicity, along with evaluating time dependent changes in its disposition, especially in presence of other drugs.